The present invention relates to an image processing method and apparatus for generating image data for recording an image by using recording materials having different densities, and a recording apparatus for recording an image by using the recording materials, each having different densities.
Printing apparatuses such as printers, copying machines, and facsimile apparatuses are designed to print an image consisting of a dot pattern on a printing medium such as a paper sheet or thin plastic plate on the basis of image information. Such printing apparatuses are classified according to their printing schemes into an ink-jet type, wire dot type, thermal type, laser beam type, and the like.
Of these types, the ink-jet type (ink-jet printing apparatus) is designed to print an image by discharging/spraying ink (printing solution) droplets from the orifices of a print head and making them adhere to a printing medium.
With the recent spread of computers, many printing apparatuses have been used, and there have been increasing demands for these apparatuses to realize high-speed printing, high resolution, high image quality, low noise, and the like. As printing apparatuses that can meet such demands, the above ink-jet printing apparatuses have become rapidly popular because they are relatively compact and capable of facilitating color printing.
As ink-jet printing apparatuses, an apparatus using a print head having an array of orifices to realize high printing speed and an apparatus having a plurality of print heads to be capable of color printing have been widely used. In consideration of the demands for high resolution and high image quality, halftone processing methods such as a dither method and an error diffusion method are used in these ink-jet printing apparatuses as methods of faithfully reproducing the gradation of image information.
According to these gradation reproduction methods, when a printing apparatus has a high resolution (1,000 dots/inch or more), excellent gradation printing can be performed. If, however, a printing apparatus has a low resolution (about 360 to 720 dots/inch), the printed dots of a highlight portion become conspicuous, and the discontinuity of pixels tends to make an image look rough. For this reason, in order to increase the number of gradation levels, a method of converting each print dot itself into a multilevel dot is used. For example, according to a known method, the voltage to be applied to a print head, the pulse width, or the like is controlled to modulate the diameter of each print dot adhering to a printing medium, thereby reproducing gradation. However, such a method is highly environment-dependent, and hence the diameters of print dots remain unstable, and there is a limit to the minimum size of a print dot that can be printed. It is therefore difficult to stably reproduce gradation.
A density modulation method is also available, in which the density of dots in a dot matrix (predetermined area) is changed while the dot size remains the same. However, a very large area is required to increase the number of gradation levels, and hence the resolution decreases.
For improving the gradation characteristics and obtaining high-density, multilevel gradation images by using such an ink-jet-printing apparatus, for example, the following methods have been proposed and put into practice: a so-called multiple droplet method of forming one dot by landing a plurality of droplets onto substantially the same portion on a printing medium, and expressing gradation by changing the number of droplets to be landed on the portion; a printing method of reproducing gradation by forming at least two types of print dots, each having different density, by using a plurality of types of ink, each type of ink has different density from each other; and a combination of these methods.
As one of the pseudo gradation reproduction means, an error diffusion method (R. FLOYD and L. STEINBERG, xe2x80x9cAn Adaptive Algorithm for Spatial Grey Scalexe2x80x9d SIDE75 DIGEST), pp. 33-37) is available.
According to Katoh, Y. Arai, Y. Yasuda, xe2x80x9cMultilevel Error Diffusion Methodxe2x80x9d (National Conference of Communication, Department in Showa 53 Year, Society of Electronic Communication in Japan (1973), pp. 504), an error diffusion method using a plurality of thresholds is realized, unlike the conventional error diffusion method using one fixed threshold. If, for example, image data ranges from 0 to 255, error diffusion is performed using xe2x80x9c128xe2x80x9d as a threshold to obtain binary data in the prior art. In contrast to this, in the xe2x80x9cmultilevel error diffusion methodxe2x80x9d by Katoh et al., when a halftone image is to be printed with two different ink densities, xe2x80x9c85xe2x80x9d and xe2x80x9c175xe2x80x9d are set as thresholds to obtain ternary print data with two different density values. Furthermore, efforts have recently been made to realize a method of obtaining multilevel data with three or more different density values and expressing a high-definition image.
As recovery means used for an ink-jet print apparatus when the print quality deteriorates, suction means and pressurizing means are used to remove foreign substances and bubbles from liquid paths by discharging ink from the nozzles of the print head. Alternatively, ink coagulations near orifices are removed by cleaning the ink discharge surfaces with a wiper. In addition, since ink adhering to the orifice surfaces of the print head upon discharging of the ink may cause a discharge failure, the orifice surfaces of the print head are wiped at a predetermined timing.
Furthermore, to discharge ink whose viscosity has increased upon evaporation of the solvent from an unused nozzle during printing operation, preliminary discharging, which differs from discharging for printing, is performed at a predetermined timing. With this operation, fresh ink is always supplied to each nozzle to perform stable printing.
In this method, however, although fresh ink is supplied to a nozzle by the above recovery operation, the solvent in the ink gradually evaporates from the nozzle tip immediately after the recovery operation, resulting in an increase in ink density in the nozzle tip with time. In the multiple droplet method or the method of expressing gradation by using several types of ink, each color of the ink having similar color and different density, in particular, the gradation balance deteriorates with an increase in ink density, and the gradation loses smoothness. As a result, pseudo contours are generated, and the image quality deteriorates in some case.
The present invention has been made in consideration of the prior art described above, and has as its object to provide an image processing method and apparatus in which, when a plurality of recording materials are to be recorded and overlaid in accordance with the density of image data, density changes of these recording materials are predicted, and data for determining recording materials to be used for recording are generated on the basis of the predicted density values, and a recording apparatus for recording an image by using these recording materials.
It is an object of the present invention to provide an image processing method and apparatus in which when a plurality of recording materials of a color which have different densities are recorded and overlaid on substantially the same portion on a recording medium to form one pixel so as to express gradation. Density changes of the respective recording materials are predicted to change the combination of recording materials to be overlaid, and a recording apparatus for recording an image by using these recording materials.
It is another object of the present invention to provide an image processing method and apparatus for predicting density changes of recording materials in accordance with an environment, and generating data for determining recording materials to be used for recording on the basis of the predicted density values, and providing a recording apparatus for recording an image by using these recording materials.
It is still another object of the present invention provide an image processing method and apparatus for predicting density changes of recording materials in the above manner, performing recording upon determining recording materials to be used for recording on the basis of the predicted density values, and performing density interpolation by distributing, to pixels around a target pixel, the error between the density value of the target pixel to be recorded and the actual density value of the image data, thereby generating image data with improved gradation reproducibility, and providing a recording apparatus for recording an image by using these recording materials.
Other features and advantages of the present invention will be apparent from the following descriptions taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.